Scientists may have figured out just what makes cancer cells so strong.

Immune cells may not even have a chance of attacking a tumor before they're warded off, a University of Pennsylvania study published Wednesday in Nature reveals. That's because cancer cells shoot off "tiny weapons" that attack immune cells before they can get close enough to fight, Stat explained Thursday.

Scientists already knew tumor cells hold a protein that inhibits immune cells' disease-fighting abilities, Stat says. But this new discovery shows cancer attacks opposing cells from afar with weapons called exomes. Because exomes can spread throughout the body and hunt immune cells down, this study could explain why cancer patients have weak immune systems — and help identify patients who will respond best to certain cancer-fighting therapies. Read more at Stat. Kathryn Krawczyk